Piketty’s strategy is to start with a panoramic reading of the data across space and time, and then work out from there. He and a group of associates, most notably Emmanuel Saez, another young French economist, a professor at Berkeley, and Anthony B. Atkinson of Oxford, the pioneer and gray eminence of modern inequality studies, have labored hard to compile an enormous database that is still being extended and refined. It provides the empirical foundation for Piketty’s argument.